The United States is monitoring a troubling Serbian military deployment along the border of Kosovo that is destabilizing the area, the White House said on Friday and called for the forces to be withdrawn.

White House national security spokesperson John Kirby told reporters that Kosovo’s peacekeeping force is going to be increasing its presence of Nato forces in northern Kosovo as a result of the tensions.

Kosovo authorities said police fought around 30 heavily armed Serbs who stormed the Kosovo village of Banjska on Sunday and barricaded themselves in a Serbian Orthodox monastery. Three attackers and one police officer were killed.

The gunbattle has prompted new international concern over stability in Kosovo, which has an ethnic Albanian majority and declared independence from Serbia in 2008 after a guerrilla uprising and a 1999 Nato intervention.

Kirby said the attack was not a random one and that the amount of types of arms that were found represent a threat not only to Kosovo personnel but international personnel, including Nato troops.

“Everyone involved in this attack needs to be brought to justice,” he said. He called on Serbian authorities to assist in the investigation into what happened.

Kirby called “a large Serbian military deployment along the Kosovo border” a destabilizing development and called on Serbia to withdraw those forces and contribute to lowering tensions.

US Secretary of State Antony Blinken earlier on Friday spoke to Serbia’s president by phone to convey US concerns, and White House national security adviser Jake Sullivan spoke with Kosovo’s prime minister to discuss the events and stressed the importance of dialogue, Kirby said.

Meanwhile, the United Kingdom’s Ministry of Defence has transferred command of an army battalion to Nato so it can provide support in Kosovo if required, it said on Friday.

“Due to the current situation in Kosovo, we have transferred command of the 1st Battalion of the Princess of Wales’s Royal Regiment – a reserve force for Nato’s Kosovo Force (KFOR) – to Nato so it can provide support if required,” they said in a statement.